The present invention relates generally to circuit breaker trip units. More specifically, the present invention relates to an electronic trip unit with adjustable sensitivity to current spikes.
The use of electronic trip units in electric circuit breakers is well known. Trip units can be used for, among other purposes, providing short circuit protection to an electrical distribution circuit. In this capacity, the trip unit samples current in the power lines of the distribution system to detect a short circuit. If a short is detected, the trip unit provides a trip signal to an actuating device, such as a trip solenoid, within the circuit breaker. Upon receiving the trip signal, the actuating device separates a pair of contacts within the circuit breaker to open the distribution circuit and protect the distribution circuit from damage caused by the short circuit.
The construction of an electronic trip unit is also known. Electronic trip units typically comprise voltage and/or current sensors, which provide analog signals indicative of the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals, which are processed by a signal processor. Electronic trip units further include RAM (random access memory), ROM (read only memory) and may also include EEPROM (electronic erasable programmable read only memory) all of which interface with the signal processor.
To detect short circuits in the distribution circuit, trip units monitor peaks in the current within the power lines. Generally, trip units compare the current in the power lines to some threshold value. For example, this threshold value may be seven times the rated current of the circuit breaker. If the current in the power lines exceeds this threshold value, indicating a short circuit, the trip unit generates the trip signal.
FIG. 1 shows a current waveform of fundamental frequency. In the waveform shown, the current peak is higher than the threshold value and, therefore, this waveform is indicative of a short in the circuit. A trip unit would generate a trip signal if the waveform of FIG. 1 were detected. FIG. 2, however, shows a current waveform with current spikes caused by high harmonic content or noise. Such current spikes can cause the circuit breaker to trip, even where no short circuit exists. Trips caused by these current spikes can be a nuisance.
Attempts have been made to overcome this problem by using processing algorithms to filter out the current spikes. While such is well suited for certain applications, such as drive systems, where current spikes are commonly generated, it is problematic in other applications, such as high-frequency systems (e.g., 400 Hz systems or resistive load circuits), where the user desires the trip unit to trip in response to such current spikes.
The above-described and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method of protection in an electronic trip unit. The method of protection includes selecting a limit value. The method further includes sensing an electrical signal to provide corresponding first and second sensed signals, each indicative of an electrical characteristic of the electric signal. The first and second sensed signals are compared to determine a rate of rise of the electrical characteristic, and the rate of rise is compared to the limit value to detect a spike in the electrical characteristic. The generation of a trip signal is withheld when the rate of rise is greater than the limit value.
In an alternative embodiment, an electronic trip unit includes a switch for selecting a limit value and a sensor for sensing an electrical signal to provide first and second sensed signals. The first and second sensed signals are indicative of an electrical characteristic of the electrical signal. The electronic trip unit further includes a signal processor responsive to the sensed signal. The signal processor has memory for storing signals including program signals defining an executable program for: comparing the first and second sensed signals to determine a rate of rise of the electrical characteristic, comparing the rate of rise to the limit value to detect a spike in the electrical characteristic, and withholding generation of a trip signal when the rate of rise is greater than a limit value.